A New Approach to Reservoirs Connectivity Analysis Using Pore Pressure Measurements in Pressure Coefficient-Depth Plots: A Case Study on Normally Pressure Reservoirs from Bohai Oilfields

Abstract

Offsets observation in pore pressure-depth plots is used to evaluate pressure compartmentalization owing to the real-time and quantitative character¬istics while drilling. However, in normal pressure and highly permeable reser¬voirs, these slight offsets between pressure gradients are hardly noticeable and therefore, uncertainties of the connectivity are frequently induced. Moreover, it fails to analyze the variation of fluid properties in reservoirs. A new approach to develop the inverse variation in pressure coefficient-depth plots by pore pressure measurements is addressed in this study. Geometric prop¬erties of this function are categorized as coincidence, monotonicity, progressivity, and symmetry. Properties of directly connected reservoirs in plots exhibits (1) overlapping inverse proportional curves, (2) a continuous trend without deviation from the inverse variation law, and function curves sharing (3) a same pair of horizontal and vertical asymptotes and (4) a unique axis of symmetry. Strictly, an absence of each will be defined as disconnecting. During our surveys on highly permeable reservoirs, the minimum offset value of 1.6 psi (0.01 MPa) reflecting the pressure seal of mudstone interval happened in two vertically stacked fluvial channel elements of KL9-X area. Prior to the recognition of compartmentalization, it is fairly a challenge to determine whether the slight offset could activate the pressure seal. But from an insight of pressure coefficient-depth plot, not having any common in geometric properties of inversely proportional function obviously gives that they are totally disconnected. Cases like this show that the accuracy to connectivity is being made an improvement in highly permeable reservoirs, through transforming the principles from identifying offsets in pressure gradients into investigating four geometric properties of inverse variation in pressure coefficient-depth plots. There was no prior work using pressure-depth plots to determine the connec¬tivity of reservoirs bearing individually different fluid properties. Only providing reservoirs with the same fluid phase, the connectivity evaluation by the linear law will continue. But the pressure coefficient-depth plot will make the connectivity analysis for different fluid phases successful because each inverse proportionate curve records a balance state of oil pool driving pressures. In BZ34-X area, it successfully managed the connectivity in a set of sandstone unit containing oil and water in two wells, respectively. Uniform geometric properties in plots prove that there is no obstructed barrier separating the unit afterwards confirmed by production performance. Conversely, the loss is arising from the incorrect deci¬sion if other methods, the linear rule for example, cannot also provide valuable suggestions. Cases studies in this article provide insights about the superiority of this con¬nectivity study, examining four geometric properties in pressure coefficient-depth plots, over classic methods as offsets observation in pore pressure-depth plots. In highly permeable reservoirs under normal pressure, sealing pressure across reser¬voir intervals is clearly recorded by differences of function properties even when the slight offset is hard to be tested in pressure gradients. Specific to the varia¬tion of fluid properties in reservoirs, breakthrough progresses were done to enable pressure measurements function in pressure coefficient-depth plots. By contrast, the new approach has been documented being a timely and accurate input while drilling. Methodologies and principles addressed in this article will aid these types of studies such as fluid (oil, gas and water) distribution, reserves acquisition, and reservoir’s appraisal and development plans optimization.